I think Booking.com was actually a client of Persuasion API; a competitor of Conversify (coincidentally all three Dutch companies.)
I think they actually ran this experiment only through their mailing list though but either way Booking.com was already using all these conversion tactics before though so at best they actually used this to occasionally turn off some of the less effective social proofs.
I think they actually ran this experiment only through their mailing list though but either way Booking.com was already using all these conversion tactics before though so at best they actually used this to occasionally turn off some of the less effective social proofs.